It all has to do with the cut plane. My guess is that you are modeling it above the cut plane. Go to Document> Floor Plan Cut Plane... and note the Cut Plane Height to Current Story setting. Then (in 3D obviously) select the wall in question and edit it's settings.
You either need to adjust the elevation of the wall to a height that it will actually be cut by the cut plane (ie. an elevation lower than the height you noted earlier), or you need to alter the way the wall is presented, but it depends on the situation. For example, you could set the wall's Show On setting to Own Story Only and the Floor Plan Display to Symbolic Cut. That pretty much ignores tha cut plane for that wall, but it also ignores the new feature of AC10 to show that wall on multiple stories - basically it mimics AC9. Or you could set the wall's Show On Setting to Automatic and the Floor Plan Display to Project with Overhead, Outlines Only or Overhead All. This will use the AC10 features but makes your wall appear as an outline only.
Clear as mud?
Cheers,
Link.